Charbonneau Anna, Novakowski Kent, Ross Nathalie
Department of Civil Engineering, Ellis Hall, Queen's University, Kingston, Ontario, Canada K7L 3N6.
J Contam Hydrol. 2006 May 30;85(3-4):212-28. doi: 10.1016/j.jconhyd.2006.02.001. Epub 2006 Mar 27.
At sites in fractured rock where contamination has been exposed to the rock matrix for extended periods of time, the amount of contaminant mass residing in the matrix can be considerable. Even though it may be possible to diminish concentrations by the advection of clean water through the fracture features, back diffusion from mass held in the matrix will lead to a continuing source of contamination. In such an event, the development of a biofilm (a thin film of microbial mass) on the wall of the fractures may act to limit or prevent the back diffusion process. The objective of this preliminary study is to explore the influence imparted by the presence of a biofilm on the process of matrix diffusion. The investigation was conducted using radial diffusion cells constructed from rock core in which biofilm growth was stimulated in a central reservoir. Once biofilms were developed, forward diffusion experiments were conducted in which a conservative solute migrated from the central reservoir into the intact rock sample. Diffusion experiments were performed in a total of 11 diffusion cell pairs where biofilm growth was stimulated in one member of the pair and inhibited in the other. The effect of the presence of a biofilm on tracer diffusion was determined by comparison of the diffusion curves produced by each cell pair. A semi-analytical model that accounts for the presence of a biofilm was used to investigate the effect of the biofilm on mass transfer due to changes in the effective porosity, effective diffusion coefficient, and the depth of penetration of the biofilm into the intact rock. The results show that the biofilm acted to plug the rock matrix, rather than forming a discrete layer on the reservoir surface. The reduction in effective porosity due to the biofilm ranged from 6% to 52% with the majority of the samples in the 30% to 50% range. Based on the present results, with more efficient biofilm stimulation, it is reasonable to assume that a more complete plugging of the microcrack porosity might be possible, leaving a much thicker and efficient barrier than could be achieved via a surface biofilm.
在裂隙岩石中，污染物长时间与岩石基质接触的区域，基质中留存的污染物质量可能相当可观。即便通过裂隙特征中清洁水的平流作用有可能降低污染物浓度，但基质中留存物质的反向扩散会导致持续的污染来源。在这种情况下，裂隙壁上生物膜（一层薄薄的微生物物质膜）的形成可能会限制或阻止反向扩散过程。这项初步研究的目的是探究生物膜的存在对基质扩散过程的影响。研究使用了由岩芯构建的径向扩散池，在中央储液器中刺激生物膜生长。生物膜形成后，进行正向扩散实验，其中一种保守溶质从中央储液器迁移到完整岩石样品中。总共在11对扩散池中进行了扩散实验，其中一对扩散池中的一个刺激生物膜生长，另一个抑制生物膜生长。通过比较每对扩散池产生的扩散曲线，确定生物膜存在对示踪剂扩散的影响。使用一个考虑生物膜存在的半解析模型，研究生物膜对传质的影响，这种影响体现在有效孔隙率、有效扩散系数以及生物膜向完整岩石渗透深度的变化上。结果表明，生物膜起到堵塞岩石基质的作用，而非在储液器表面形成离散层。生物膜导致的有效孔隙率降低范围为6%至52%，大多数样品在30%至50%的范围内。基于目前的结果，通过更高效地刺激生物膜，合理推测有可能更完全地堵塞微裂纹孔隙，形成比表面生物膜更厚且更有效的屏障。
